The present invention relates to a system of multi-coupled PWM-controlled power converters (inverters) which reduces the harmonic components of output voltage and suppresses circulating currents flowing between the PWM-controlled power converters, and also relates to a control method for the same.
There is a conventional power conversion system which reduces the harmonic components of output voltage, as disclosed in Unexamined Japanese Patent Publication No. Sho 60-98875 (to be referred to as "Citation 1" hereinafter). The Citation 1 is a method of controlling a multi-coupled power conversion system including voltage-type PWM-controlled power converters of n in number (n is an integer of two or greater) connected in parallel, with interphase reactors being provided between common-phase output terminals of the power converters, and it is characterized by the shifting in steps of 360/n degrees of the carrier signals which control individual power converters. Namely, the carrier signals that are the base signals for producing the PWM waveform of the parallel-connected power converters are given phase differences of 360/n degrees in correspondence to the power converters, thereby reducing the harmonic components of the output voltage.
However, although the prior art of Citation 1 is capable of reducing the harmonic components of the output voltage, it has the problem of a smaller effect of harmonic reduction for line voltages that are the composition of the phase voltages. Because of this, the harmonic components of output current created in proportion to the line voltage cannot be reduced sufficiently, and in the case of driving a motor, for example, a torque ripple and speed ripple are created and the motor cannot rotate stably.
Although the prior art disclosed in Citation 1 is capable of reducing the harmonic components of the output voltage, there arise lateral currents flowing between the output phases of the power converters, so that the motor supply current decreases and large output interphase reactors are needed.
There is another conventional power conversion system described in Unexamined Japanese Patent Publication No. Hei 5-211775 (to be referred to as "Citation 2" hereinafter). This is a serial or parallel multi-coupled power conversion system having its output voltage controlled by pulse-width modulation, in which a zero-voltage vector adjacent to the command output voltage vector and having a zero line voltage, a neutral-voltage vector of a varying neutral voltage or a neutral-voltage vector of a varying neutral-voltage is selected based on the amplitude and phase of the command output voltage vector, with the order of selection of the three voltage vectors in the specified period being controlled so that the zero-voltage vector or neutral-voltage vector comes first.
The prior art of Citation 2 is a means of preventing the loss of harmonic reduction of the output voltage, which is the feature of the serial multi-coupled power conversion system. This is in contrast to the usual 2-level power conversion system which produces two positive or negative voltages, in which case, when the command output voltage has its amplitude increased, the zero output voltage period decreases and the positive (output voltage: E) or negative (output voltage: -E) period increases, resulting in an increased amplitude of output voltage. The citation 2 does not deal with the state of a smaller amplitude of output voltage, i.e., this is not a power conversion system which reduces the harmonic components irrespective of the amplitude of output voltage.
Unexamined Japanese Patent Publication No. Hei 5-56648 (to be referred to as "Citation 3" hereinafter) discloses a parallel operation controller for a PWM power conversion system. The controller includes output current detectors for individual power converters, a circulating current calculator which calculates the circulating currents of the power converters based on the detected output currents, and a parallel operation inhibition circuit which produces and feeds a base cutoff signal to one power converter when a circulating current exceeds a certain value.
However, the prior art disclosed in Citation 3 has a problem in that when the base cutoff operation takes place, the output current decreases to the point where it is unable to drive a load such as a motor.